The present invention generally concerns thermoplastic elastomer compositions comprising blends of chlorinated polyethylene and a crystalline olefin polymer. Especially concerned are vulcanized chlorinated polyethylene blended with crystalline polyolefins and blended with thermoplastic chlorinated polyethylene. The present invention also concerns preparation of said compositions by dynamic vulcanization or by a sequential combination of dispersion of the vulcanized material and dispersion of the remaining chlorinated polyethylene. The present invention further concerns the use of a non-peroxide cure package to accomplish said dynamic vulcanization.
Thermoplastic elastomers, which can be processed and fabricated by methods used for thermoplastics and do not require vulcanization to develop elastomeric properties, are known (see, for example, U.S. Pat. No. 3,265,765 as well as Hartman et al., "Butyl Grafted to Polyethylene Yields Thermoplastic Elastomer," Rubber World, Oct. 1970, pp. 59-64).
Dynamic vulcanization is a process whereby a blend of plastic, rubber and rubber curative is masticated while curing the rubber. The term "dynamic" indicates the mixture is subjected to shear forces during the vulcanization step as contrasted with "static" vulcanization wherein the vulcanizable composition is immobile (in fixed relative space) during vulcanization. One advantage of dynamic vulcanization is that elastoplastic (thermoplastic elastomeric) compositions may be obtained when the blend contains the proper proportions of plastic and rubber. Dynamic vulcanization processes are described in U.S. Pat. Nos. 3,037,954, 3,806,558, 4,104,210, 4,116,914, 4,130,535, 4,141,863, 4,141,878, 4,173,556, 4,207,404, 4,271,049, 4,287,324, 4,288,570, 4,299,931, 4,311,628 and 4,338,413.
Known dynamic vulcanization processes are believed to be somewhat unsuitable for making soft compositions because as the rubber level rises the resulting compositions become less fabricable. In other words, the compositions give poor extrudates and, sometimes, cannot be extruded at all. Accordingly, there is a need for processes for preparing soft, extrusion-fabricable, thermoplastic elastomeric compositions.
U.S. Pat. No. 4,130,535 discloses thermoplastic vulcanizates or blends of polyolefin resin and monoolefin copolymer rubber which are processable in the same manner as thermoplastics even though the rubber is fully cured. The thermoset state is avoided by simultaneously masticating and curing the blends. The blends comprise about 25-95 percent by weight of the resin and about 75-5 percent by weight of the rubber. Oil extended vulcanizates have a ratio of 35 to 65 percent of the resin and about 65 to 35 percent of the rubber. Peroxide, azide and sulfur vulcanizing agents may be used to effect curing of the rubber. Typical monoolefin copolymer rubbers include saturated EPM (ethylene-propylene rubbers) or unsaturated EPDM (ethylene-propylene-diene terpolymer rubbers).
U.S. Pat. No. 4,594,390 teaches that improved thermoplastic elastomer materials are obtained when a composition comprising polypropylene, an EPDM rubber, an extender oil and a curative is masticated at a shear rate of at least 2000 sec.sup.-1. Suitable results are obtained with shear rates of 2500 to 7500 sec.sup.-1.
U.S. Pat. No. 4,207,404 discloses thermoplastic elastomer compositions prepared by dynamic vulcanization of blends of chlorinated polyethylene and nylon in the presence of a peroxide vulcanizing agent.
U.S. Pat. No. 3,806,558 discloses partially cured blends of a monoolefin copolymer rubber, such as those disclosed in U.S. Pat. No. 4,130,535, and a polyolefin plastic, usually polyethylene or polypropylene. The blend is mixed with a small amount of curative, and subjected to curing conditions while working the mixture dynamically.
A. Y. Coran, R. P. Patel and D. Williams, in an article entitled "Rubber-Thermoplastic compositions. Part V. Selecting Polymers for Thermoplastic Vulcanizates," Rubber Chemistry and Technology, Vol. 55, 116 (1982), describe approximately one hundred thermoplastic vulcanizate compositions, based on nine kinds of thermoplastic resin and eleven kinds of rubber. All compositions contain sixty parts of rubber and forty parts of plastic. They prepare these compositions by melt mixing the plastic, rubber and other components in a Brabender or Haake mixer. Generally, the plastic, rubber and other components of the composition, except for curatives, are mixed at controlled elevated temperatures (Table I) for about 2-6 minutes during which time the plastic melts and a blend is formed with the rubber. After blend formation, curatives are added to crosslink the rubber, and mixing is continued until a maximum consistency or mixing torque is observed. Each composition is removed from the mixer and then remixed for an additional minute in the molten state to insure uniformity of the mixture. One of the rubber materials is chlorinated polyethylene (CPE). The plastic materials, listed in Table I on page 117, include polypropylene (PP), polyethylene (PE), polystyrene (PS), an acrylonitrile-butadiene-styrene polymer (ABS), a styrene-acrylonitrile copolymer (SAN), polymethyl methacrylate (PMMA), poly-tetramethylene terephthalate (PTMT), Nylon-6,9 (PA) and polycarbonate (PC). One of the mechanical properties, tension set (ASTM D412-66), is determined by stretching 51 mm long specimens to 102 mm for 10 minutes then by measuring set after 10 minutes relaxation. Chlorinated polyethylene compositions are cured by peroxides, specifically 2,5-dimethyl-2,5-bis(t-butyl peroxy)hexane. Tension set values abstracted from Table IX at page 125 are as follows: (a) CPE/PP - 55%: (b) CPE/PE - 58 %: (c) CPE/ABS - 65%: (d) CPE/ABS - 91%: (e) CPE/PMMA - 82%: (f) CPE/PTMT - 40%: (g) CPE/PA - 59 %: and (h) CPE/PC - 85%. They note that the 40% value may not be accurate because the CPE is insufficiently stable to withstand processing at the high melt temperatures for PTMT.
One of the applications for chlorinated polyethylene vulcanizate blend compositions is in single-ply roofing membranes. However, microbial action causes cracking or "alligatoring" in chlorinated polyethylene-based membranes. While thermoplastic chlorinated polyethylene is easily fabricable in sheet form, it tends to crack. While thermoset or vulcanized chlorinated polyethylene are more resistant to microbial cracking, it is less easily fabricable. Therefore, a blend of the two would be advantageous for blending the features of both. However, it is not possible to partially crosslink or vulcanize one chlorinated polyethylene in the presence of another without also affecting the other.